Thermally sensitive power elements using capillary tubes and having vapor/gas fills are commonly used to operate electric switches or other control devices in response to upward and downward changes in temperature of a space or area to be controlled. The use of the capillary tubes permits location of the electric switch or other control device at a point which is protected from adverse ambient conditions of temperature, or humidity. However, if the capillary tubes traverse regions where the temperature ranges below the normal temperature ranges encountered in the controlled space or area cross ambient temperature affects can readily produce control point errors.
It is a characteristic of the usual vapor/gas fills used in thermally sensitive power elements of the capillary tube type that the effective working internal pressure developed is dictated by the region or point in the system subjected to the coldest temperature. Thus if the capillary tube in its traverse from the controlled area to the diaphragm motion producing portion is subjected to lower temperature than those encountered in the controlled area, the cut-in and cut-out temperatures at which the associated electric switch or other control device responds may be considerably offset from those desired.
Various expedients to minimize such cross ambient affects have been tried, including use of thermal shields and heaters with the portions of the capillary tube extending between the controlled areas and their associated diaphragm power elements. However, such expedients have not proved to be too satisfactory in minimizing such cross ambient temperature affects, and have added to the cost.
It is also a characteristic of the vapor/gas fills used in such thermally sensitive power elements that they exhibit greater temperature sensitivity at one portion of their working temperature range than others. For use in refrigeration and air conditioning where the differential between desired cut-in and cut-out may be substantial, it is very desirable to have approximately the same temperature sensitivity, but this has been found hard to come by with present day forms of thermally sensitive power elements that use vapor/gas fills. Mixtures of different commercially available refrigerant vapor/gases are usually used to provide a mixture that will provide reasonably satisfactory temperature sensitivity at both ends of the working temperature range. However, that expedient has not proved too satisfactory, as there can be adverse interactions between the gases, particularly over extended periods of time.